To date a number of on-line monitoring methods have evaluated quality in partially oriented yarns (POY) or fully drawn yarns (FDY) yarn spinning processes. One such method is that of the on-line measurement of the denier regularity. Another method is that of tension measurement between the spinneret and the first godet. Although in many processes there may be a reasonably stable tension in this zone with a minimum of disturbing factors, nevertheless in more and more processes oil dispersing air jets are fitted in this zone, creating a source of significant tension variations and disturbance. The main component of force in this zone is a variable air drag on the filaments prior to them being formed into a yarn at the first contact point--the oil applicator guide, but there are other influences that reduce the value of this tension measurement as indicative of the morphological aspects of the spun yarn. These include the friction of the filament bundle with the oil and guide surfaces, the effect of the air jets if fitted and the draw force of the yarn. As an alternative method, the yarn tension may be measured in other zones, for example between the first and second godet and between the second godet and the take-up, but these arrangements have also failed to provide methods of obtaining useful information due to the large tension variations in these areas. Between the godets it is usual to fit air jets which cause substantial tension variations. After the godets the tension is cyclically variable because of the change in yarn length due to the traversing of the yarn as it is forwarded onto the package, the tension variation frequency being twice that of the traverse. Typically a 0.5 to 1.0% yarn path length change occurs during the winding process. In addition, to prevent patterning during the winding process, the traversing of the yarn is usually subjected to a periodic disturbance, which causes a variation in the change of yarn path length. Because of the above described problems, to date none of the above described methods has been commercially successful.
It is now well known that a key parameter indicative of the quality of POY and FDY is the "draw force". This is the tensile force at which the extension of the yarn increases considerably for substantially no increase in that force. The draw force, to which the initial modulus of the POY or FDY is directly related, is a very small factor influencing the tension in the yarn in the zone prior to the first godet, and is swamped by the accepted variations from the other sources. As a consequence, it has been effectively impossible to obtain useful information about draw force from a tension measurement in this zone. Hence to date, such a measurement has been made under laboratory conditions as a routine quality control test, but in such a way that only a very small sample from a small percentage of the packages produced can be tested in practice. In consequence there has been a requirement for a method of monitoring this parameter, or a closely related parameter, on-line so that the quality of 100% of all yarn produced can be evaluated.